As this kind of combustion apparatus, there is known an apparatus in JP-A 2017-78542 having the following features. In other words, the combustion apparatus includes: a combustion plate flange part disposed on a perimeter of the combustion plate in a manner to project outward beyond an inner rim of the body flange part and beyond an inner rim of the combustion box flange part, and also in a manner that an outer rim of the combustion plate flange part is positioned inside an outer rim of the body flange part and inside an outer rim of the combustion box flange part; a packing having a portion interposed between the combustion plate flange part and the body flange part, and a portion interposed between the combustion box flange part and the body flange part outside the outer rim of the combustion plate flange part, such that the combustion box flange part is connected, with the packing being sandwiched therein, to the body flange part. According to this arrangement, the combustion plate flange part comes into direct contact with the combustion box flange part. Further, in that portion of the combustion box which lies between the burner and the heat exchanger, there is disposed cooling means such as water jackets and the like. Therefore, it is possible to release the heat of the combustion plate through the combustion plate flange part to the combustion box flange part to which the cold by the cooling means is transmitted.
However, since the combustion plate becomes considerably high in temperature during combustion, the temperature of the combustion plate flange part becomes high even in the above-mentioned construction. Therefore, the packing to be used must be of an expensive one with a high heat-resistance.